1. Field of the Invention
The present invention relates to a method for the manufacture of a conical tubular member, and a member manufactured according to the method.
2. Description of the Prior Art
For a number of different applications, there is a need for tubular elements, having a conically reduced cross-section in the longitudinal direction, for example for use as poles and masts, flag-poles, and for many other fields of application. Such tubular members, having a substantially cylindrical cross-sectional configuration, can be manufactured by rather expensive manufacturing methods, e.g. by means of a drawforming operation in connection with a draw plate having a variable diameter.
Within many fields of application, it is not of primary importance that a conically tapered tubular member is arranged with a substantially circular cross-sectional configuration, but other cross-sectional configurations are acceptable, and for certain applications also more desirable. The present invention relates to such tubular and conical members, which include longitudinally extending embossings or corrugations. Conical tubular members of this type have previously been manufactured in various fashions, and a first example is disclosed in British Pat. No. 7,754 of 1902, according to which patent a rectangular and plane blank first is arranged with substantially parallel corrugations extending from one edge portion, and with a successively reduced depth in direction from said edge portion. These corrugations change the shape of the plane blank in such a way, that it thereafter can be bent into a conical and tubular member. An alternative method of manufacture is based on the use of a cylindrical and tubular member as basic material, and that embossing rollers produce longitudinally extending corrugations or embossings in the tubular material, and examples of this manufacturing method are shown in British Pat. No. 1,462,370 and French Pat. No. 1,260,814. Previously known methods of manufacture are thus based on two alternative blanks, either a plane blank, which in plane condition is arranged with parallel embossings having a succesively decreased depth, or a cylindrical and tubular member, which is brought into contact with embossing tools to accomplish longitudinally extending corrugations or embossings, when said tools are moved along the cylindrical and tubular member. To use a pre-shaped plane blank can be regarded as an acceptable method of manufacture, when the conical and tubular member to be manufactured has a relatively small length, and preferably also a relatively large angle of taper. However, to use a cylindrical tubular member as a blank, and to use embossing rollers which when moved in longitudinal direction of the member also succesively move towards the center axis of the member, is a method of manufacture that requires extremely complicated and thus also expensive machinery equipment, and it is also impossible to accomplish an end product, in which opposed sides of embossed or corrugated portions contact each other, unless the corrugated tubular member in a final operation is made subject to pressure applied against the outer surface in order to further reduce the diameter. A further problem is the spring return force of the material, and the difficulties in accomplishing substantially U-shaped embossings, having sharp corner portions at the bottom surfaces of the corrugations, and having side and bottom surfaces extending substantially plane.